Billions of years ago the universe was formed in the Big Bang. From out of this explosion emerged all the matter we now see in our universe, but it was mostly in the form of particles like electrons and protons. As the universe expanded and cooled, these particles formed the first atoms in a vast, thick gas that filled all of space.
After a few million years, this gas began to clump together because small regions of this gas got to be dense enough that the force of gravity could pull it together even though the rest of the universe was still continuing to expand. By this time, the universe was completely dark to the naked eye with not a single star shining through the depths of space. We call this time the Dark Ages. The universe was only 1/100 the size that it is now.
Within a few hundred million years, these clumps continued to form and in some cases collide. This built up bigger collections of matter and also triggered tremendous bursts of star formation activity. The first stars in the universe exploded as supernova within a few million years. The larger collections of matter eventually became the modern galaxies around us today. We can actually see some of this galaxy-building activity going on in some of the deepest images we have of the distant universe. Remember, as we look out into space, the light we see is more and more ancient, and shows what things looked like long ago, not today!
In time, galaxies also formed groups and clusters along with their local neighbors. Today, our Milky Way galaxy is one member of a group of 33 galaxies we call the Local Group. There are also many other clusters of galaxies that we have charted and some of them contain thousands of members. In fact the nearest one to us is called the Virgo Super Cluster and it will completely swallow our Local Group cluster in another five to ten billion years.
What this all has to do with our discovery is that astronomers have known for some time now that this clustering of matter didn't just happen over night. First smaller clusters formed billions of years ago, then over time, some of these gained other galactic members as their gravity grew in strength. Clustering of galaxies in the universe grew in scale from only a handful of members to thousands of members in the present-day universe. It is very hard to spot individual galaxies because they are very faint, but we can see their combined light as it shines down through the eons.
The Two-Micron Sky Survey was an ambitious program to 'photograph' the entire sky in the infrared part of the light spectrum just beyond the reddest color you can see in a rainbow. The neat thing is that, because the universe is expanding, light gets stretched as it travels through space. The light leaving distant infant galaxies that would have looked normal to the human eye back then, is now shifted into the infrared from where (and when) we are now seeing this light. So actually, if you want to study the distant, ancient universe and what galaxies looked like 10 billion years ago, you have to use infrared telescopes and infrared data. That was one of the things that the 2MASS program wanted to provide astronomers between 1997-2000.
What we did was to use a special collection of their data that was normally only used to 'calibrate' their survey sky images. A typical image in their survey only has data collected for about 8 seconds. This is like working in a dim room with a camera and using a 'time-exposure' setting. Well, 8 seconds is good enough to reveal millions of stars in our Milky Way and many nearby galaxies, but it is not long enough to study the distant universe, so no one bothered to think about doing this kind of research. As it turns out, the calibration data, when you add it all up, can give you exposures of 3000 to 4000 seconds for a few special directions in the sky. That is more than enough to reveal the deep universe out to at least 5 billion light years and more.
So what we did was to add this data together for one particular patch of the sky in the constellation Hercules. It wouldn't have been our first choice for a cosmic hunting ground, but it turned out to be good enough!
Because it is a technically difficult thing to accomplish, we decided not to go after the so-called Cosmic Infrared Background directly. This has been attempted by many other researchers with only limited results. Instead, we took advantage of the fact that any dim light in the photograph that had to do with distant galaxies would look clumpy across the sky. Distant galaxies would not be seen individually, but their combined glows should be easily seen in the 2MASS data, or so we hoped. As it turned out, we did see something!!!
There is a neat mathematical calculation you can do that tells how much information there is in a picture. You take a specific 'pixel' in the image and then multiply its brightness by the brightness of all pixels that are at different distances from it. You then take the next pixel and do the same thing, and the next, and the next. After you have done this for all the millions of possible pairs in the picture, you add the numbers together sorted by the distance. You then graph this information. If the picture is just random 'noise' you will see a rather flat line, but if there is some kind of information in the scene, it will have a more complicated shape. For our study, we knew that distant galaxies and their light should be clumped in a particular way that would have a very unique fingerprint in this kind of plot. What we found after analyzing the 2MASS image was just this fingerprint!!
There is a lot of 'junk' in the photograph we produced, in addition to the dim light of the infant universe. Could this junk be fooling us? The biggest source is the atmosphere of the earth itself. Then there is the dim glow of dust in our solar system called the Zodiacal Light. There is also the light from hundreds of stars within our own Milky Way in the direction we were looking, and the individual glows of light from nearby galaxies. We had to get rid of each of these, or prove to ourselves that they really didn't matter.
Because the data was gathered over 5 months time, when it was combined, any atmospheric effects canceled out on average and this reduced their strength greatly. Also, when we compared the light in a short-exposure image against the light in our final deep image in all three wavelength bands, we discovered that the atmosphere seen in the short image had a very different color to it that what remained in the deep image. This was proof that what we had detected had little to do with clumps in the atmosphere. Besides, there is no physical reason why such atmospheric clumps should persist over a 5 month time! Take a look outside and you will see how variable clouds can be over time!
We removed all the stars in the field by simply punching them out of the image and allowing a generous margin all around the edges. When our program had finished, the 90% of the sky that remained in the image had no discernible 'point sources' remaining. This method also deleted nearby galaxies too, because if a galaxy like our Milky Way was closer than about 2 billion light years, you would be able to see it as one of these sources, and also it would look slightly elliptical.
The Zodiacal light which fills our inner solar system is a problem, but there is no evidence that it is anything more than a featureless cloud of dust at the size scales we are studying. Also, at the time of the year and direction we were looking it is not very bright. More importantly, the color of this light looks like the color of our sun because it is reflected sunlight .When we looked at the color of the light we were seeing, it didn't match the zodiacal light at all, so this means what we are seeing can't have anything to do with interplanetary dust.
So, we have definitely detected something that produces clumpy light in the distant universe, but there is a second, and very exciting, thing about it. There is about 2-3 times more of this light that we can expect from galaxies as the evolve normally over the last 10 billion years. We think we are seeing the glimmerings of a still more distant population of light sources, and this might include the first generations of stars shining at us down through the eons of time. To be seen at our wavelengths, the stars must have left their dusty nurseries already or the light would have been absorbed and turned into light at an even longer wavelengths.
In the near future, we will be looking at more of these 2MASS fields and see if the 'signal' we are seeing persists across the sky, and under many different viewing conditions with the telescopes and atmosphere.
The fun has actually just started, and it makes up for the 6 hard years of work and the many disappointments we have had. We were funded by a $325,000 grant from NASA which we won back in 1995, and hope to continue this work with a grant from the National Science Foundation should our research proposal submitted in November 2001 be accepted. Cross your fingers!!!